This invention relates to sulfur recovery systems and processes. In particular, it relates to a process and apparatus for increasing the amount of energy recovered from the effluent gas of the thermal reactor of the sulfur recovery plant.
Federal government restrictions on the emissions of sulfur oxides and other pollutants have led to the installation of sulfur recovery facilities throughout the United States. One method widely used by the industry for the elimination of elemental sulfur from a gaseous stream is the Claus process. The Claus process follows the general formula of: ##STR1## In a typical Claus sulfur recovery unit, hydrogen sulfide and air are combined in a thermal reactor where a portion of the hydrogen sulfide undergoes combustion to sulfur dioxide. The sulfur dioxide thus formed then reacts according to the above equation to form elemental sulfur.
In most sulfur recovery plants, the thermal reactor is a muffle furnace. The furnace includes a metal shell which encloses the combustion zone. The shell is lined on the inside with a refractory which insulates the shell from the high temperatures of the combustion zone. Generally, a sufficient amount of refractory is used to maintain the shell at a temperature no higher than about 600.degree. F. To conserve energy the shell of the thermal reactor is usually at least partially enclosed by an aluminum jacket. The aluminum jacket is positioned around the shell of the thermal reactor so that there is an air gap of several inches between the metal wall of the reactor and the aluminum jacket. In order to allow for air convection between the jacket and the shell, the aluminum jacket has openings at its top and bottom. This construction causes air from below the aluminum jacket to flow through the opening at the bottom, then vertically upward through the gap into the opening at the top. This air flow maintains the shell of the thermal reactor at the desired temperature.
In spite of the losses of substantial amounts of heat, air convection through the jacket is necessary in order to maintain the temperature of the shell at or below about 600.degree. F. Without it, the temperature of the shell would rise to above 600.degree. F. causing damage to the shell. It has, therefore, been assumed that energy loss through the shell of the thermal reactor cannot be avoided.
The view of the industry's experts on this subject is summarized by J. H. Barnhart in "Energy Analysis of a Claus Plant: Many of the Obvious Ways of Saving Energy in a Claus Plant Are Frequently Overlooked," Chemical & Engineering Progress, pp. 58-59, May 1978:
One place where heat loss to the atmosphere cannot be avoided is the shell of the thermal reactor. In spite of using adequate internal refractories and insulation, the shell of this vessel must not be insulated to the point where the metal temperature is above 600.degree. F., for to exceed this temperature causes metal deterioration by sulfiding. Many a thermal reactor has been damaged by overzealous application of insulation.
The problem which was believed to be unsolvable was commercially significant because the loss of energy associated with the shell of the thermal reactor is by no means minute. In a plant size of 17 LTD, with a furnace shell of approximately 5 feet in diameter by 24 feet long, air passes through the gap between the thermal reactor and aluminum jacket at approximately 2030 SCFM. Air which has an initial temperature of 90.degree. F. is heated to a temperature of approximately 150.degree. F. by radiant heat from the reactor shell. The resulting heat losses are about 140 MBTU/hour.
The present invention solves the problem which was believed to have no solution. It substantially eliminates the heat loss from the shell of the thermal reactor without raising the temperature of the shell or adversely affecting the operation of the reactor.
Accordingly, one object of the present invention is to provide a method for conserving the energy dissipated through the metal shell of the thermal reactor.
Another object of the present invention is to provide a more efficient and economical sulfur recovery unit.
A further object of the present invention is to provide a method for recovering the heat emanating from the shell of a thermal reactor to improve the efficiency of the thermal reactor.
Still another object of the present invention is to practically eliminate the heat loss to the atmosphere from the thermal reactor without damaging the thermal reactor.
A further object of the present invention is to increase the reaction temperature in the thermal reactor.
Still another object of the present invention is to increase the amount of steam produced in, and exported from, the sulfur recovery plant.
A still further object of the present invention is to provide a more cost effective process for recovering sulfur utilizing the Claus process.
Other objects of the present invention will occur to those skilled in the art upon study of this disclosure.